1. Field of the Invention
The present invention is directed to certain improvements in vehicle brake mechanisms and, more specifically, to a brake shoe and an anchor pin for a vehicle brake assembly.
2. Description of the Prior Art
The invention is particularly useful in a rotary cam actuated brake assembly which includes a pair of brake shoes having adjacent ends pivotally mounted to a support and a rotatable actuating cam disposed between the other ends of the brake shoes. Rotary movement of the cam causes each of the brake shoes to pivot outwardly about its pivot or anchor pin to contact the inwardly facing friction surface of the brake drum. This type of brake assembly is used for heavy duty brake applications which may employ a dual or a single web brake shoe. The brake shoes may be cast or fabricated. The brake shoes may also carry a roller type cam follower on the ends adjacent the actuating cam. The opposite adjacent ends of the brake shoes may be mounted for pivoting movement about a single anchor pin as disclosed in U.S. Pat. No. 3,398,814 or each shoe may be pivotally mounted for movement about one of a pair of spaced anchor pins as disclosed in U.S. Pat. No. 3,114,437.
Dual web brake shoes for the brake assemblies employing a pair of spaced anchor pins are provided with axially aligned, laterally extending bores in each of the webs at the end of the shoe to be pivotally mounted to the support. The brake shoes are mounted to the assembly by aligning the web bores on opposite sides of a bore through a boss formed on the brake support or spider and inserting a pin axially through the spider bore and the coaxial brake shoe bores. The pin, generally referred to as an anchor pin, may be press fit to the boss or locked against rotation by a set screw or other means. Snap rings may be provided to the axially extending ends of the pin to prevent substantial axial displacement of the anchor pin relative to the support. The foregoing arrangement fixes the brake shoe against substantial movement relative to the support except for the desired pivoting movement about the anchor pin. In this type of brake assembly the anchor pin must be withdrawn from the spider bore when its becomes necessary to remove the brake shoes for the purpose of replacing or relining the same.
This is not necessary in brake assemblies of the type where the brake shoes are mounted for pivoting movement about a single anchor pin. In that type of assembly the adjacent ends of the brake shoe webs are each recessed to provide arcuate bearing surfaces usually less than 180.degree. in arc, which are seated on opposite sides of the anchor pin by a radially directed movement relative to the pin. The open ended recesses provided to the brake shoe ends permit assembly to and disassembly from the anchor pin without moving the anchor pin relative to the spider. Although this arrangement provides for ease of assembly and disassembly, the brake shoes are not positively secured to the anchor pin and may become self-actuating. That is, when the brake assembly is actuated, the cam causes the brake shoes to pivot about the anchor pin to bring the friction lining pads into contact with the brake drum and thereby inhibit rotation of the brake drum and the wheel to which it is attached. The friction force between the brake drum and the linings secured to the brake shoes tends to draw the linings into further contact with the brake drum and thereby increase the magnitude of the friction force. The effect the friction force will have on the linings depends upon the direction of rotation of the brake drum.
The friction force between the drum and the lining secured to one of the brake shoes will be effective in the same general direction as the force applied by the cam actuator and will compliment the brake actuating force. The friction force between the drum and the lining secured to the other brake shoe will be opposite to the direction of the force applied by the rotatable cam actuator. The friction force at the circumference of that brake shoe will tend to move the brake shoe away from the anchor pin and toward the actuating cam at the other end of the shoe.
Under static conditions there is virtually no space available to accommodate any movement of the brake shoe other than the intended pivotal movement into contact with the friction surface of the brake drum. However, the brake assembly distorts under dynamic braking conditions. The system pressure and temperature generated during braking cause the brake shoes and the brake drum to expand radially in those areas where the pressure and temperature increase is the greatest. Drum type brake assemblies which are normally depicted as comprising a plurality of brake shoes with friction lined tables of generally cylindrical curvature and closely surrounded by a cylindrically curved friction surface of a brake drum under static conditions become elliptical or oblate under dynamic braking conditions. In a rotary cam actuated brake assembly the brake drums and brake shoes expand radially about the mid-point of the brake shoes and afford additional clearance for the anchor ends of the brake shoes to move away from the anchor pins if the brake shoe webs are not secured against such movement. Movement of the brake shoe away from the anchor pin causes undesirable noise and chatter as well as unnecessarily rapid wear of the lining on the brake shoe most often subject to this type of "self-actuation".
Although the components of the brake assembly could be made more massive to produce an assembly which is less susceptible to distortion under dynamic braking conditions, such an assembly would be economically unfeasible because of the additional metal required in the manufacturing process and the mass added to the weight of the vehicle. The economics of manufacture and vehicle operation thus dictate that the state of the art rotary cam actuated brake assembly is both satisfactory and reliable and should be improved wherever possible to enhance its operation and servicability.